1. Field of the Invention
The present invention relates to a method of analysis of the distribution of concentration in a substrate, more particularly relates to a method of analysis of the distribution of concentration in a semiconductor substrate which enables an accurate analysis of not only the distribution Of concentration of specific atoms in the depth direction of the semiconductor substrate, but also the distribution of concentration in the horizontal direction of the semiconductor substrate.
2. Description of the Related Art
The SIMS method of analysis offers a ultrahigh sensitivity (down to the ppb level) and a high depth-resolution (down to 1 nm). The SIMS method is being taken note of in the field of semiconductors, for example, as the most effective method of analysis when determining the distribution of concentration of doping impurities in ultra shallow regions resulting from ultra low energy ion implantation.
In the SIMS method, a finely focused primary ion beam is irradiated on the surface of a sample to drive out atoms from inside the sample. The group of ionized particles forming part of this, that is, the secondary ions, are analyzed in mass spectrometer. Normally, the energy of the primary ions used is several keV to 20 keV or so where the sputtering efficiency is high. The primary ions used are O.sub.2.sup.+, Cs.sup.+, Ar.sup.+, Ga.sup.+, etc.
The lateral resolution of SIMS instruments, however, is heavily dependent on the operating principles of the instrument and the conditions of the analysis. Even when finely adjusting the factors of the instrument well, it is at most several microns to 10 or so microns. Therefore, it has been almost impossible to analyze the concentration of impurities distributed in a region of less than the submicron size, such as the lateral spread from an ion implanted region, directly at the surface from above the sample.
It has been proposed to perform some sort of processing when using the SIMS method to analyze the concentration of impurities distributed in regions of less than the submicron size. The methods tried in the past may be roughly grouped into the following two types:
The first type of method is the method described in J. Vac. Scl. Technol. B, Vol. 10, No. 1, Jan/Feb. 1992, pp. 353 to 357. In this method, a plurality of parallel etched grooves are formed by dry etching so as to cross at an extremely fine angle (.theta.=0.08.degree. to 0.1.degree.) the longitudinal direction of a plurality of stripe-like ion-implanted regions. The depth of each groove is made somewhat deeper than the main region of distribution of the implanted impurities. Next, to avoid a shape effect at the time of SIMS analysis, use is made of semiconductor planarization to bury the grooves by a material such as polycrystalline silicon. The original surfaces of the implanted substrate adjoining the edges of the grooves of the sample obtained in this way are analyzed point-wise by successively shifting the ion beam or the sample stage in the direction of the grooves.
In this method, dependent upon the cross angle .theta., the real distance in the lateral direction .DELTA.X is projected magnified along the edge lines of the etched grooves and the resolution is improved. For example, it is increased about 800- to 1000-fold when 1=.DELTA.X/sin.theta. and .theta.=0.08 to 0.1.
In this first method, however, even if the ion beam is focused to a spot of at most 10 .mu.m, which is the practical effective diameter, a lateral resolution of only 0.01 to 0.0125 .mu.m can be obtained when converted to real distance in the lateral direction. For example, if .DELTA.X=0.1 .mu.m, when projected, it becomes 1=80 to 100 .mu.m. The analysis points are at most split into 8 to 10 points.
Further, in the first method, processing of the samples requires sophisticated and complicated semiconductor processing techniques such as photolithography, dry etching, and planarization and, further, time is taken for the preparation of the samples.
Further, in the first method, it is difficult to judge with a high degree of precision the cross points between the boundaries between the ion-implanted regions and non-implanted regions and the stripes of the etched grooves. In the end, it is difficult to maintain a high absolute positional precision for the analysis of the distribution of concentration in the lateral direction. Further, even assuming analysis under the identical conditions at different analysis points, accurate determination of the initial position of depth from the surface is extremely difficult.
The second method is the method proposed by R. von Criegern et al. of Siemens Co. In this method, the side face of a sample in which ions have been implanted is cut in a direction substantially perpendicular to the surface of the sample and SIMS depth profiling is performed perpendicularly with respect to this face. This second method as well enables measurement of the distribution of impurities caused by ion implantation in the lateral direction since it calls for SIMS analysis from the side face of the sample.
However, in this method, it is necessary to cut and polish the side face of the sample, so special tools are required for the polishing.
Further, in the second method, since SIMS depth profiling is performed, a sub-nanometer level (less than 1 nm) of resolution is required, so it is not possible to use a point-like ion beam. Beam scanning such as raster scanning of a certain surface region (for example, 100.times.100 .mu.m or 500.times.500 .mu.m) is essential. Accordingly, despite the fact that the doping impurities form a certain gradient of concentration with respect to the direction of ion implantation even directly below the mask in the same way as the region directly under the ion implantation windows, it is not possible to resolve and capture that state of distribution. That is, in the conventional second method, the state of distribution of concentration of the impurities diffused and distributed in the lateral direction was just integrated with respect to the direction of implantation.
Note that there are other methods of analyzing the distribution of concentration other than the SIMS method.
The level of concentration of impurities diffused and redistributed from the ion implantation to a region under the mask is dependent on the implanted dosage, the annealing conditions, etc. but is about 10.sup.13 to 10.sup.17 atoms/cm.sup.3. Even directly under a mask edge at a depth of the projected range (Rp), it is at most no more than 10.sup.19 atoms/cm.sup.3. Accordingly, with physical analytical techniques other than the SIMS method (XPS, AES, RBS, EPMA-WDX/EDX, etc.), the concentration is below all of their detectable limits and therefore analysis is impossible. Methods tried in the past other than analytical techniques have been the transmission electron microscope (TEN) method, the scanning tunnel microscope (STM) method, etc. Each of these methods evaluate the pn junction regions from the perpendicular cross-sections. The features and problems of these will be pointed out briefly below.
In the method of selective etching and TEM examination of the cross-section, the perpendicular cross-section of the implanted region is chemically etched for selective etching dependent on the level of concentration of the impurities. Naturally, continuous contours of concentration are formed along the regions of both the implantation windows and implantation mask. It is possible to examine this pattern of etching at a high magnification by a transmission electron microscope.
This technique requires sophisticated technology in fabricating ultra thin film samples for the TEM examination and further it is difficult to handle the results of the examination quantitatively. It is superior as a method of qualitative examination however.
In ultrahigh vacuum STM measurement using a scanning tunnel microscope, the pn junction surface is sliced open and exposed under an ultrahigh vacuum (UHV) (or in the atmosphere) and the STM chip is scanned two-dimensionally. The in-plane resolution of the probe is a comparatively high 10 nm, but the quantitative relationship between the tunnel current and the impurity concentration is not yet clear.